It is known that when scanning and reconstruction are not done following the beat of the heart in heart region imaging, a pseudo-image called a motion artifact and blurring occur, which is unsuitable for clinical diagnosis. It is conceivable to hasten as much as possible the scanning speed as one approach to solve this problem, and there is a CT apparatus that uses an electron beam to realize this. This CT apparatus has a scanning speed of about 100 ms and is capable of tomography that is as sharp as if the heart had stopped. However, the cost of this CT apparatus using an electron beam is high, the CT apparatus is large, it is necessary to dispose the CT apparatus in addition to an existing X-ray CT apparatus, and there is the potential for the CT apparatus to become a burden in terms of equipment investment.
Thus, an ECG-gated imaging technique has been proposed which is based on existing X-ray CT apparatuses. This is a technique which successively collects projection data across plural heartbeats on the same slice plane, uses as a reference R waves of electrocardiograph information recorded at the same time, sets the time and heartbeat phase width thereafter, collects only the projection data of the same cardiac time aspect from the data among the plural heartbeats, and reconstructs a tomogram with ECG reconstruction means.
The technique of obtaining an aimed tomogram by extracting the necessary data from among the wealth of projection data when later creating a tomogram in an optional cardiac time aspect in this manner is commonly called retrospective segmented reconstruction. However, retrospective segmented reconstruction has the problem of an increase in patient exposure due to redundant measurement. In order to accommodate this, there is a technique called prospective which systematically acquires projection data necessary for tomogram creation. Because this technique determines in advance the cardiac time aspect to be acquired and applies X-rays aimed only at that range, excess exposure can be avoided.
JP-A-2001-190547 discloses technology that similarly reduces exposure and images moving organs such as the heart with an X-ray CT apparatus. This technology includes a rotating mechanism that rotates/drives an X-ray tube in a state where an object to be examined is sandwiched between the X-ray tube and an X-ray detector, an X-ray control unit that supplies power for applying the X-rays to the X-ray tube, and a reconstruction device into which projection data detected by the X-ray detector are inputted to reconstruct a tomogram. -The object transmittance thickness changes in accordance with the rotation of the X-ray tube and the detector. An increase in noise resulting from needless invalid exposure and insufficient radiation dose occur. If control to reduce the tube current is not done when the object transmittance thickness is short, there is the potential to increase superfluous exposure. Conversely, if control is not done so that the tube current becomes larger when the object transmittance thickness is large, there is the potential to increase image noise. Thus, JP-A-2002-263097 discloses the transmittance thickness-dependent control shown in FIG. 2c. Namely, the tube current is varied in accordance with the rotational angle ″ and the body axis direction position z from the scanogram. With this technique, the X-rays applied to the object can be greatly reduced because the tube current is controlled in consideration of the transmittance thickness of the object.
Here, it is an object of the present invention to provide an X-ray CT apparatus which, in regard to a periodical motion portion such as a heart, considers both the time aspect during the periodical motion and the X-ray transmittance thickness in the object, pursues reduction of invalid exposure and image quality improvement, is a developed form of prospective scanning and a reconstruction method that also enables retrospective reconstruction, and simultaneously achieves a reduction in the object's exposure to X-rays, an improvement in diagnostic image quality, and an insurance of the freedom of post-imaging reconstruction.
In X-ray CT apparatuses employing the conventional retrospective ECG-gated imaging technique, abnormalities in the motion of the ventricular walls and abnormalities in the coronary artery can be observed because a tomogram of the ventricular wall diastole is taken, but once the heart region has been imaged, the projection data are combined by ECG reconstruction means and image reconstruction processing is conducted. Thus, an image of the heart cannot be observed while imaging the heart region.
Thus, it is another object of the present invention to provide an X-ray CT apparatus that is configured to develop the prospective scanning method and the reconstruction method, reduces the motion artifact stemming from the movement of motion portions such as the heart, and with which an image of the motion portion can be observed in real-time while imaging that motion portion.